In the United States and many other countries, heart disease is a leading cause of death and disability. One particular kind of heart disease is atherosclerosis, which involves the degeneration of the walls and lumen of the arteries throughout the body. Scientific studies have demonstrated the thickening of an arterial wall and eventual encroachment of the tissue into the lumen as fatty material builds upon the vessel walls. The fatty material is known as "plaque." As the plaque builds up and the lumen narrows, blood flow is restricted. If the artery narrows too much, or if a blood clot forms at an injured plaque site (lesion), flow is severely reduced, or cut off and consequently the muscle that it supports may be injured or die due to a lack of oxygen. Atherosclerosis can occur throughout the human body, but it is most life threatening when it involves the coronary arteries which supply oxygen to the heart. If blood flow to the heart is significantly reduced or cut off, a myocardial infarction or "heart attack" often occurs. If not treated in sufficient time, a heart attack often leads to death.
The medical profession relies upon a wide variety of tools to treat coronary disease, ranging from drugs to open heart "bypass" surgery. Often, a lesion can be diagnosed and treated with minimal intervention through the use of catheter-based tools that are threaded into the coronary arteries via the femoral artery in the groin. For example, one treatment for lesions is a procedure known as percutaneous transluminal coronary angioplasty (PTCA) whereby a catheter with an expandable balloon at its tip is threaded into the lesion and inflated. The underlying lesion is re-shaped, and hopefully, the lumen diameter is increased to improve blood flow.
In recent years, a new technique has been developed for obtaining information about coronary vessels and to view the effects of therapy on the form and structure of a site within a vessel rather then merely determining that blood is flowing through a vessel. The new technique, known as Intracoronary/Intravascular Ultrasound (ICUS/IVUS), employs very small transducers arranged on the end of a catheter which provide electronic transduced echo signals to an external imaging system in order to produce a two or three-dimensional image of the lumen, the arterial tissue, and tissue surrounding the artery. These images are generated in substantially real time and provide images of superior quality to the known x-ray imaging methods and apparatuses. Imaging techniques have been developed to obtain detailed images of vessels and the blood flowing through them. An example of such a method is the flow imaging method and apparatus described in O'Donnell et al. U.S. Pat. No. 5,453,575, the teachings of which are expressly incorporated in their entirety herein by reference. Other imaging methods and intravascular ultrasound imaging applications would also benefit from enhanced image resolution.
Transducer backing materials having relatively low acoustic impedance improve signal quality in transducer assemblies comprising PZT or PZT composites. The advantages of such backing materials are explained in Eberle et al. U.S. Pat. No. 5,368,037 the teachings of which are expressly incorporated in their entirety herein by reference. It is also important to select a matching layer for maximizing the acoustic performance of the PZT transducers by minimizing echoes arising from the ultrasound assembly/blood-tissue interface.
When designing a very small device for manufacture in large quantities it is important to take into consideration practical limitations such as manufacturability, reliability, resiliency and performance. The ultrasound catheter assembly must produce high quality raw image signals for the signal processing system located outside the body within which the intravascular ultrasound transducer assembly is inserted for imaging. However, there is an interest in limiting the number of parts since added complexity can increase the manufacturing costs and reduce the yield of the intravascular ultrasound catheter assemblies. The devices must be sufficiently resilient to withstand handling during manufacture and use.